The present invention relates generally to sensors, and in particular to a new class of electrical strain sensors.
Most prior art electrical strain sensor elements are based upon piezoresistive materials in which the obstacles to charge transport change in magnitude with applied strain. Such materials commonly exhibit an essentially linear change in resistance with applied stress, and typically show little or no change in resistance with the magnitude of the electrical field applied to measure the resistance of the element.
There are many forms of piezoresistive sensors known n the prior art. The most common such sensors, commonly applied in strain gauges and pressure sensors, utilize doped polysilicon or crystalline silicon piezoresistive elements.
The sensitivity of a piezoresistive element to an applied strain can be expressed as the gauge factor, which is the fractional change in resistance per unit strain xcex5, or xcex94R/xcex5R. While metals typically exhibit gauge factors between 2 and 4, silicon piezoresistive elements commonly exhibit gauge factors having magnitude as large as 200.
The importance of having a large gauge factor in practical applications is a direct result of the small strains associated with commonplace stresses. The elastic limits of most materials appear at less than 0.01 strain, and are often additional orders of magnitude smaller.
For example, assume one desires to measure an applied pressure of 1 megapascal (roughly 150 pounds per square inch (psi)) with a resolution and accuracy of 1%. In typical metals and semiconductors, the corresponding strain is on the order of 10xe2x88x925, the relevant modulus being on the order of 100 gigapascals. If the gauge factor is 3, a typical value for metallic strain gauges, one is faced with the challenge of measuring a change in resistance of 3xc3x9710xe2x88x925 with a resolution and accuracy of 3xc3x9710xe2x88x927 of the total resistance, which is a very difficult task. If the gauge factor is 100, a value more typical for doped silicon, the change in resistance is 10xe2x88x924, and the resolution and accuracy required is 10xe2x88x926, which are measurement goals still difficult, but more easily attained.
However, in many cases the change in resistance associated with measuring a desired level of stress is still smaller than designers are comfortable with, even with the large gauge factor of a silicon strain gauge. Designers often resort to strain gauges which incorporate mechanisms or structures whose function is to concentrate the stress from a large area upon a smaller piezoresistive element, thereby increasing the resulting strain, and the change in resistance for a given applied stress.
Such structures, however, commonly lack sufficient physical robustness for many applications. Also, application of large strains often increases the susceptibility of materials to chemical attack, hence limiting the environmental robustness of concentrator stress gauges. Further, the limited gauge factors which are attainable even using doped silicon piezoresistive elements render even stress gauges incorporating a stress concentrating structure insufficiently sensitive for many applications.
There therefore exists in the prior art need for an electrical strain element and sensor which are more sensitive than those of the prior art. An additional need is for more robust electrical strain sensors which still retain sufficient sensitivity for general application.
The present invention addresses the above needs by applying newly discovered physical phenomena to produce a new class of piezoconductive strain sensors.
An advantage of the new piezoconductive strain sensors is that gauge factors are attainable which are orders of magnitude larger than those of prior art piezoresistive strain sensors.
An additional advantage of the new piezoconductive strain sensors is that their physical and environmental robustness generally exceeds that of prior art piezoresistive strain sensors.
These and other advantages of the process of the present invention will become evident to those skilled in the art.
A new class of electrical strain sensor has been invented, based upon the newly discovered strain dependence of Poole-Frenkel charge transport. A Poole-Frenkel piezoconductive strain sensor comprises a dielectric material containing carrier traps. An electric field is applied to this material to assist thermal excitation of charge carriers from the traps into the conduction band of the material. The depth of such traps is proportional to the mechanical strain applied to the dielectric material, so that the conductivity of the material, changes with the applied strain. Poole-Frenkel piezoconductive strain sensors exhibit sensitivity to applied strain as much as two orders of magnitude larger than do conventional piezoresistive strain sensors.